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<head>
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  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>

  <!-- h1/p/div -->
  <!-- span/a/img/strong -->

  <div>
    <h1></h1>
    <p></p>
    <a href=""></a>
    <img src="" alt="">
  </div>

  <!-- 特殊: 不要在p元素中放div元素 -->
  <p>
    123
    <div>我是div元素</div>
    abc
  </p>

  <!-- 行内级元素中不要放块级元素 -->
  <span>
    321
    <div></div>
    <p></p>
    cba
  </span>

</body>
</html>